Elemental sulfur autotrophic partial denitrification (S0-PDN) with high pH and free ammonia control strategy for low-carbon wastewater: From performance to microbial mechanism

Heterotrophic partial denitrification has been recognized as an efficient nitrite supply technology for anaerobic ammonia oxidation (ANAMMOX), but it cannot eliminate the requirement for organic carbon. In this study, six elemental sulfur-packed bed reactors were operated under high pH and free ammonia (FA) conditions to achieve elemental sulfur autotrophic partial denitrification (S0-PDN) without organic carbon requirement. Long-term performance demonstrated stable NO2−-N accumulation (≥89.5%) with optimal pH of 8.5–9.5 and FA concentrations of 36.7–146.9 mg/L in the reactors. The NO3−-N conversion was minimally affected with effluent NO3−-N concentrations of 35.8–41.4 mg/L and effluent NO2−-N concentrations of 52.1–62.1 mg/L. However, achieving S0-PDN at low FA concentrations (≤6.1 mg/L) caused a serious decrease in NO3−-N conversion due to high pH (10.0). The success of S0-PDN primarily relied on the inhibition of NO2−-N reduction by high FA ensured by NH4+/NH3 buffer system. Metagenomic analysis revealed a decrease in microbial diversity and functional species. The relative abundance of sulfur autotrophic denitrification species in partial denitrification (PDN, 2.0%) was lower compared to complete denitrification (CDN, 3.0%), with Thiobacillus denitrificans and Thiobacillus thioparus being the dominant microorganisms. However, the nitrite reductase genes were enriched in PDN. The mechanism of S0-PDN may be the effect of high FA on the synthesis or activity of nitrite reductase, resulting in a higher activity ratio of nitrate reductase to nitrite reductase in PDN (>5.0) compared to CDN (<4.0). S0-PDN provides an economical NO2−-N supply technology for ANAMMOX, potentially promoting significant cost reductions in nitrogen removal for low-carbon wastewater treatment.